One significant problem with heat pumps is a possible system malfunction whereby the thermostat for the space to be heated and/or cooled by the heat pump commands compressor operation so as to either heat or cool the space but the compressor either does not operate or, in some cases, cycles on and off. Another possible system malfunction is where the compressor is energized and running but is not compressing the refrigerant; this can occur because of compressor valve failures and/or the loss of refrigerant. There are usually no obvious indications of these faults to a person located near the thermostat because the compressor is typically located remote from the thermostat. With many systems this can means (when the thermostat is calling for heating of the building) that auxiliary electric resistance heating is automatically used to heat the building, i.e., a backup heating system; however, this usually results in a much higher cost of heating. Accordingly, various prior art schemes have been devised for attempting to detect whether or not the compressor is running, or is running without pumping refrigerant in the system, but all of these prior art arrangements have one or more shortcomings. For example, one prior scheme is to use the pressure of the refrigerant at the discharge side of the compressor; however, this does not provide a reliable enough signal. Also, it has been proposed that the value or magnitude of the electric current and/or electric voltage energizing the motor driving the compressor be monitored; however, these schemes only indicate that the motor is being powered and do not confirm that the compressor is actually pumping refrigerant.
Another prior art arrangement is that set forth in the co-pending patent application of Dale A. Mueller and Stephen L. Serber, Ser. No. 954,266, filed Oct. 24, 1978 U.S. Pat. No. 4,246,763, wherein compressor fault detection is provided by monitoring the difference between the temperature of the discharge of the compressor and the temperature of the outside or outdoor air. The present invention is similar to that disclosed in said co-pending application of the applicants but is an improvement thereover in that the outdoor air temperature sensor is replaced by an outdoor coil temperature sensor which offers several technical advantages and also a cost advantage, all of which will be explained in more detail below.
An object of the present invention therefore is to provide a significantly improved compressor fault detection system for a reverse cycle refrigeration system.